These patches contain a completely rewritten blktap implementation
and are an open source release of what Citrix intends to use in future
releases of XenServer. They contain a number of bug fixes from my
previous patchset.

They also contain Citrix's implementation of the VHD image format. VHD is
what XenServer uses to store file-based images, and this code is
considerably more robust and efficient than the qcow implementation that
is in the tree today.

Benefits to blktap2 over the old version of blktap:

* Isolation from xenstore - Blktap devices are now created directly on
the linux dom0 command line, rather than being spawned in response
to XenStore events. This is handy for debugging, makes blktap
generally easier to work with, and is a step toward a generic
user-level block device implementation that is not Xen-specific.

* Improved tapdisk error handling and memory management. No
allocations on the block data path, IO retry logic to protect guests
transient block device failures. This has been tested and is known
to work on weird environments such as NFS soft mounts.

* Pause and snapshot of live virtual disks (see xmsnap script).

* VHD support. The VHD code in this release has been rigorously
tested, and represents a very mature implementation of the VHD image
format.

* No more duplication of mechanism with blkback. The blktap kernel
module has changed dramatically from the original blktap. Blkback
is now always used to talk to Xen guests, blktap just presents a
Linux gendisk that blkback can export. This is done while
preserving the zero-copy data path from domU to physical device.

These patches deprecate the old blktap code, which can hopefully be
removed from the tree completely at some point in the future.

In addition to the above patches, the (external) ioemu git repository
currently has some very heavy dependencies on blktap, and does not like
the change of the blktap directory name to deprecate that code. The
following patch needs to be applied to fix ioemu:

Ideally we can come up with a better integration between blktap2 and ioemu
than what is currently in place with the old tapdisk code. For instance,
blktap2 makes it possible to just treat the tapdisk-based devices as raw
devices for qemu, severing these (now unnecessary) dependencies.